The present invention relates generally to an apparatus and method for thermoforming plastic articles and, in particular, to a thermoforming apparatus designed to form an undercut portion in the plastic article.
While thermoforming plastic articles from a heated sheet of plastic material may have production advantages over molding articles by the so-called injection molding process, one advantage of utilizing the injection molding process has been the ability to produce plastic articles in relatively complex shapes. For example, with injection molding, it is possible to produce relatively deep undercuts in various portions of a molded article. However, one of the practical limitations of the injection molding process relates to the minimum wall thickness which can be obtained in the molded article. The production of relatively thin walled articles by injection molding is difficult, if not impossible, to achieve. Also, the production rate of an injection molded process is far less than that of a thermoforming process.
By utilizing a thermoforming process, it is feasible to produce plastic articles such as containers, lids, and other plastic articles having relatively thin walls. As compared to an injection molding process, a thermoforming process has the advantage of relatively high speed production and considerably more economical tooling costs. However, the thermoforming technique has had the disadvantage of not adapting itself to rapid molding of complex shapes and, in particular, it has been difficult to efficiently produce a thermoformed article having an undercut portion.
It has been possible to produce undercuts in a plastic article by the thermoforming process by the use of expanding collets or split mold assemblies. However, the use of expanding collets or split molds greatly increase the complexity and tooling cost of the machinery. More importantly, expanding collets and split molds greatly increase the length of the production cycle of the plastic article.
There have been several developments directed to producing an undercut portion in a thermoformed plastic article. One such development includes the use of cooled mold portions which impinge upon selected areas of a heated plastic sheet and rigidify the same so that the uncooled portions can be folded to form an undercut or reverse bend section. Such an approach is disclosed in more detail in U.S. Pat. No. 3,284,553.
Another approach to producing undercut portions in a thermoformed plastic articles is disclosed in U.S. Pat. No. 3,337,664. This patent discloses the use of differential air pressure across selected portions of the heated plastic sheet to cause an undercut portion to be formed in the plastic article.
Still another approach to forming undercut portions in thermoformed plastic articles is disclosed in U.S. Pat. No. 3,126,582. In this patent, a sheet of plastic material is vacuum formed around a mold member carrying a piece of resilient material which projects beyond the edge of the adjacent mold section to form the undercut portion, and which flexes to facilitate stripping of the formed article from the mold without the need of having a movable mold member.
One approach to forming undercut portions in an injection molded article is disclosed in U.S. Pat. No. 4,383,819. This patent discloses an injection mold which utilizes movable undercut forming elements which project into the mold cavity and are simultaneously moved outwardly from the mold cavity as the molded product is removed from the mold cavity. However, this mold cannot be used in a thermoforming operation.